docs: def save_dot(self, file_name=None):
          '''
          Saves the current graph representation into a file
          '''
  
          if not file_name:
              warnings.warn(DeprecationWarning, "always pass a file_name")
              file_name = self.temp_dot
  
          fp   = open(file_name, "w")
          try:
              for chunk in self.iterdot():
                  fp.write(chunk)
          finally:
              fp.close()